The Supreme Court seemed ready Wednesday to block a California law that would require euthanizing downed livestock at federally inspected slaughterhouses to keep the meat out of the nation's food system.

The court heard an appeal from the National Meat Association, which wants a 2009 state law blocked from going into effect. California barred the purchase, sale and butchering of animals that can't walk and required slaughterhouses under the threat of fines and jail time to immediately kill nonambulatory animals.

But justices said that encroached on federal laws that don't require immediate euthanizing.

"The federal law does not require me immediately to go over and euthanize the cow. Your law does require me to go over and immediately euthanize the cow. And therefore, your law seems an additional requirement in respect to the operations of a federally inspected meatpacking facility," Justice Stephen Breyer told a California lawyer.